The invention relates to a spacer for a long substrate in the interior of a long tube, of the type wherein a superinsulation material is arranged between the substrate and the tube, and to a coaxial tube system with such a spacer.
The invention is based on the following considerations:
Optimal thermal insulation in refrigeration technology is achieved by multi-layer foil insulation in a high vacuum (superinsulation). The total heat flow is brought to what is currently the lowest technically possible value by minimizing the components Qgas, QFK and Qrad.
The heat loss flows Qgas are reduced by evacuating the insulation space.
The solid heat bridges, e.g., spacers, which are responsible for QFK, are structurally reduced by minimizing the solid-state contact flows.
The radiation losses Qrad are reduced by metallizing the interior walls or by installing highly reflective foils.
QFK plays a decisive role, especially in flexible, vacuum-insulated cryogenic conduits. To prevent direct contact at any point along the course of the conduit between the cold interior tube and the exterior tube, which is at room temperature, spacers of various designs have heretofore been used. These spacers must be capable of transmitting the largest possible force component but must likewise have the property of low thermal conductivity. The spacers of the prior art have the drawback that they do not implement these two necessary goals equally well. If low thermal conductivity was achieved, the mechanical loading capacity was low; if high mechanical loading capacity was achieved, very high thermal conductivity had to be accepted.
German publication DE-C2-2 136 176 discloses a tube system consisting of two concentric tubes, e.g., an electrical cable operated at low temperatures or a conduit for transporting liquid or gaseous heated or cooled media, whose interior tube is held in position inside the surrounding exterior tube by spacers, which are made of little material. The spacers are seated at intervals on the interior tube and are supported against the inner surface of the exterior tube. The spacer consists of a plurality of slotted rings, whose slot width corresponds to the diameter of the interior tube. On each ring, a thread element is fastened in the slot area. The rings are interconnected in such a way that they can be pivoted about a common axis of rotation. When the spacer is placed onto the interior tube, the thread elements partially wrap around the interior tube. The exterior tube is supported against the outer circumference of the spacer formed by the rings. This design requires very little material but a relatively large amount of space within the insulation area. Since it is furthermore difficult to fix the thread elements permanently to the rings and the fixation can be loosened under tensile loads, this spacer is less suitable for flexible tube systems where tensile or compression loads of more than 10,000 N may occur during bending.
U.S. Pat. No. 2,914,090 discloses a spacer for a coaxial tube system. The spacers are seated on the interior tube spaced apart from each other at a longitudinal axial distance. They hold the interior tube at a distance from the exterior tube. Each spacer consists of an interior metal tube section enclosing the interior tube and an exterior tube section, which is likewise made of metal and contacts the interior wall of the exterior tube. Between the interior and the exterior tube sections, three spoke-like struts spaced equidistantly across the circumference are provided and welded to both the interior and the exterior tube section. The interior tube section is made of two coaxial tubes between which thermal insulation, e.g., made of asbestos, is inserted.
Although this spacer has great mechanical strength it lacks the necessary insulating properties.